The use of a visible transmitting infrared reflecting coating on the envelope of an incandescent lamp to reflect infrared energy back to the filament to raise its operating temperature and thereby reduce the amount of energy consumed by the filament to a desired temperature is known. A typical approach is to use optically precise spherical envelopes and a compact filament which is located at the optical center of the envelope. A lamp of this type is shown in U.S. Pat. No. 4,160,929, granted July 10, 1979, which is assigned to the assignee of this invention. Using such an approach, energy savings in excess of 50% with the coating of the foregoing patent in a spherical envelope is theoretically attainable. This corresponds to an increase in luminous efficacy in excess of about 2 times that attainable in a normal lamp operating at the same filament temperature.
From a practical point of view, it is not possible to make either a point source or a spherical filament. In general, it has been determined that an elongated filament, either coiled-coil or triple coiled which is either horizontally or vertically mounted with respect to a spherical envelope is the most practical embodiment of lamp to be made with an infrared reflecting coating. However, when an optically precise spherical reflector is used in conjunction with a non-spherical, such as an elongated, filament some of the radiation returned from the reflector on the envelope is lost due to aberration at the ends of the filament. Once this radiation is lost on one reflection, it is essentially lost for all subsequent reflections from the envelope reflective coating unless some type of recovery mechanism is employed.
The present invention relates to an improved incandescent lamp in which the envelope is ellipsoidal in shape and has a coating for reflecting infrared energy. The filament is in the form of an elongated cylinder which is centered inside of the reflecting envelope. The ellipsoidal envelope is designed such that its two foci are located on the axis of the filament and at predetermined distances from each end of the filament to reduce the aberrational losses. By using this approach, the aberrational losses can be reduced to about half that of a sphere enclosing the same cylindrical filament. Also, the use of an ellipsoidal element concentrates the returned IR radiation at two points of predetermined distances from the ends of the filament rather than at a single point. This makes the temperature gradient more uniform.
It is, therefore, an object of the present invention to provide an incandescent lamp whose envelope has an infrared reflective coating means in which the envelope is in the shape of an ellipsoid.
Another object is to provide an incandescent lamp with an ellipsoidal envelope having its foci lying along the axis of an elongated filament.
A further object is to provide an incandescent lamp whose envelope is in the shape of the ellipsoid, with the ellipsoid design such that the two foci of the ellipsoid are located at a predetermined distance from each end of the filament.